The present invention relates to hydrostatic power steering devices of the type used to control the flow of fluid from a source of pressurized fluid.
The present invention may be used in connection with various types of hydrostatic power steering devices, including fluid controllers of the type used to control the flow of fluid to a fluid pressure actuated device, such as a steering cylinder which comprises part of a full-fluid-linked vehicle steering system. The invention may also be utilized in connection with a torque-generating device of the type wherein a relatively low-torque steering input results in a relatively high-torque output, which then serves as a Power input to a mechanical steering device, such as a rack and pinion steering system.
A typical steering device of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter, a valve means, and an arrangement for imparting follow-up movement to the valve means, in response to the flow of fluid through the fluid meter. Typically, the valve means of such devices includes Primary (spool) and follow-up (sleeve) valve members, which are displaced (deflected) relative to each other by rotation of the vehicle steering wheel to define variable flow control orifices therebetween. The flow through the steering device is generally proportional to the area of the variable flow control orifices, with the area of the orifices in turn being proportional to the rate at which the steering wheel is rotated, and therefore, proportional to the rotational displacement between the spool and sleeve.
Although the typical steering devices of the type described above have performed in a generally satisfactory manner, and have been commercially successful for a number of years, the above-described relationship between flow control orifice area and rotational displacement between the spool and sleeve have somewhat limited the capability of such devices. For example, it has typically not been possible to control or vary any of the individual steering functions in response to any input, other than steering wheel rotation. By way of example only, it would be desirable in certain vehicle applications to have the load reaction capability of a fluid controller increase with increasing vehicle speed, and/or to have the aggressiveness (gain rate) of the steering decrease with increasing vehicle speed.